The present invention relates to devices for determining the composition of a test material and more specifically devices that use multiple fluorescence techniques for determining composition.
X-ray Fluorescence (xe2x80x9cXRFxe2x80x9d) is one method for quantifying the elemental distribution of materials, but as the sought-for element gets lighter, XRF becomes less sensitive. As a general rule, XRF is useful for all elements heavier than about titanium. In special cases, XRF is effective for measuring the fractional weight of lighter elements; for example, sulphur in oil is easily quantified. However, the method ceases to have any quantitative value for carbon, oxygen, fluorine and sodium and other very light elements. To give a useful measure of these and other light elements, the preferred in situ method is optical spectroscopy.
Optical spectroscopy is a known technique for determining elemental and chemical compositions. Almost all portable optical spectroscopy systems currently available for measuring the elemental composition of alloys use a spark discharge to excite the optical spectrum to be analyzed. In recent years, lasers have been used to induce plasmas that result in fluorescent optical and near ultra-violet spectra. Variations of the basic technique involve different lasers and different spectrometers. The formed plasma may be of millimeter or micron size and the optical spectra may be viewed over microseconds or time-resolved in nanoseconds. The general technique is often referred to as laser-induced breakdown spectroscopy or LIBS, though sometimes it is referred to as laser-induced photon spectroscopy or LIPS.
New lasers and new modalities are regularly being developed so that the new acronym LIPF xe2x80x9claser induced photon fluorescencexe2x80x9d is more appropriate. LIPF applies to any laser method for inducing photon spectra, from the infrared to the near ultra-violet, which results in the identification of the elements or compounds in any sample matrix.
In LIPF processes, molecules/atoms are excited to higher electronic energy states via laser absorption and subsequently fluoresce; the intensity of this fluorescence is, in general, a function of the species concentration (number density), and the gas temperature and pressure. Among other things, this fluorescence is linearly dependent on the absorber number density. By virtue of the fact that the energy states of molecules/atoms are quantized, the spectral absorption regions are discrete; however, for large molecules, the spacing of the discrete transitions may be sufficiently small (and the number of transitions sufficiently great) that discrete absorption regions are not observed (only absorption bands are observed). Typically, single-interaction fluorescence occurs at wavelengths greater than or equal to the laser wavelength, and again for atoms and diatomic molecules especially, discrete fluorescence transitions may be observed. LIPF while producing single-interaction fluorescence, more generally produces a high-temperature plasma in which atoms are excited to higher energies than the energy of the laser photons so that lower wave-length transitions are observed. For more information regarding LIPF see Romero et al.,xe2x80x9cSurface and tomographic distribution of carbon impurities in photonic-grade silicon using laser-induced breakdown spectrometryxe2x80x9d Journal of Analytical Atomic Spectrometry, Vol. 13, June 1998 which is incorporated herein by reference in its entirety. See also Hwang, xe2x80x9cA feasibility study of micro-laser induced breakdown spectroscopy applied to lead identification in metal alloys and environmental matricesxe2x80x9d Thesis (S.M.) Massachusetts Institute of Technology, indexed (OcolC)48198136 (1998) which is attached hereto as appendix A and is incorporated by reference herein in its entirety.
LIPF has the potential to measure the distribution of almost all elements in any matrix. In practice, the method has commercial sensitivity for a subset of elements, though with the proper choice of laser, that subset can encompass the most important light elements in a given application. LIPF, however, has the general drawback that the efficiency of production of the optical emissions, that is, the intensities of the induced spectral lines, depends strongly on the matrix and the measuring conditions. Comparison standards are essential.
Although both LIPF and XRF are known techniques, the techniques have not been combined into a single device to produce a more complete composition of a test material. Further, the measurements have not been graphically combined and scaled to provide a spectral representation on a graphical display device.
In a first embodiment of the invention there is provided a device for identifying the composition of a target sample. The target sample may be a matrix such as a metal alloy, a soil sample, or a work of art. The device includes an x-ray fluorescence detector that produces an x-ray signal output in response to the target sample. The x-ray fluorescence detector is sensitive to elements above a particular threshold. In general the threshold element is titanium. The device also includes an optical spectroscope that produces an optical signal output in response to the target sample. In one embodiment, the optical spectroscope is a laser induced photon fluorescence detector. The laser induced photon detector is sensitive to the lighter elements below the threshold that typically include sodium, carbon and oxygen. Further, a processor is included that analyzes and combines the x-ray signal output and the optical signal output to determine the composition of the test material. The processor receives the output signals from the x-ray fluorescence detector and the laser induced photon detector and begins to analyze the data. The analysis determines the type of material that is being processed, such as, a metal alloy that is an aluminum alloy. The processor then compares the data from the output signals concerning the common element and then scales the optical signal output data to produce a displayable output that contains the concentrations of elements within the test material. In one embodiment, the x-ray fluorescence detector, the optical spectroscope and the processor are contained within a single housing. In other embodiments, the x-ray fluorescence detector and the optical spectroscope are not in the same housing, yet share a common processor.
In general, the data that is contained within the optical signal output is relative data concerning the concentrations of elements in the test sample, while the data that is contained within the x-ray signal output is absolute. To produce an output signal which can be displayed and which provides data regarding the concentration of elements in the test material, both the laser induced photon fluorescence detector and the x-ray fluorescence detector are sensitive to at least one common element within the target sample. The processor uses data from the optical signal output and the x-ray signal output about the common element to normalize data contained within the optical signal output.
In another embodiment, the laser induced photon detector and the x-ray fluorescence detector are positioned within the device so that both analyze a common area of the target sample. The two detectors within the device may operate simultaneously in one embodiment to produce their respective output signals that are transferred to the processor. In another embodiment, the x-ray fluorescence detector operates first. In the preferred embodiment, the length of time that it takes to measure the test material is shorter than the time for removal of the test material from the device and insertion of the test material in a second device for analysis. Further, the device can be sized to be both portable and battery operated.